Hitherto, as a gas separation membrane, a cellulose acetate membrane has been well known. However, the cellulose acetate membrane is insufficient in chemical resistance, thermal resistance, etc. Therefore, it cannot be said that the cellulose acetate membrane has a practically sufficient performance. Moreover, as a membrane whose thermal resistance property is improved, a polysulfone semipermeable membrane is industrially manufactured, but it also is insufficient in permeation performance. Moreover, a silicone membrane is well known as a selective permeation membrane of oxygen. However, silicone cannot provide a sufficient mechanical strength and be industrially satisfactory.
Recently, researches and developments of a polyimide resin separating membrane excellent in strength, heat resistance and gas selective permeability have been conducted.
Journal Polymer Science Polymer Physics (J.polym. Scie. Polym. Phys., 28, 2291 (1990)) describes an aromatic polyimide membrane having high gas permeability and selectivity, which is made from a phenylenediamine component in which one ortho position amine functional group is replaced by an alkyl group. Moreover, U.S. Pat. No. 4,717,394 and Japanese Patent Application Tokkai No. Sho 63-123420 describe an aromatic polyimide membrane having high gas permeability, which is made from phenylenediamine in which all ortho position amine functional groups are replaced by alkyl groups.
However, a polyimide having higher permeation performance and high separation selectivity and a gas separation membrane using such polyimide are still demanded.